Visual indicator for radio blind approach systems



J. D. WESTON- Oct. 29, 1946.

VISUAL INDICATOR FOR RADIO BLIND APPROACH SYSTEMS Fild May 4. 1945 I vntor m Pfi? Attorney Patented Oct. 29, 1946 UNITED VISUAL INDICATOR FOR RADIO BLIND APPROACH SYSTEMS J efi rey Dennis Weston, London, England, assignor to Standard Telephones and Cables Limited, London, England, a British company Application May 4, 1943, Serial No. 485,614 In Great Britain May 11, 1942 8 Claims. 1

This invention relatesto a method of and arrangements for distinguishing visually between two signals of the same rectangular wave form but whose variations are of opposite sign and is particularly applicable to visual indicating arrangements for radio blind approach systems of the kind in which an approach path is defined by the intersection of two overlapping lobes, complementary signals being transmitted by the two radiations so that the signals received on the approach path constitute a continuous dash, or no apparent signal.

One indicatingarrangement for this type of system utilises a dynamometer type of indicating instrument. Such an arrangement is described in British Specification No. 526,114. The present invention aims at provid'mg an indicating arrangement utilising a direct current moving coil indicating instrument. Previously known indicating arrangements for blind approach systems of the type specified which utilise direct current moving coil indicating instruments depend for satisfactory operation upon a balance between two rectifiers or two thermionic valves. Such a balance is generally intolerable, since a slight disturbance of it, which may be caused by atmospheric or other changes gives riseto a false indication.

According to the present invention the method of distinguishing visually between two signals of the same rectangular wave form but Whose variations are of opposite sign comprises developing from either signal a comparison signal of constant sign and combining this comparison signal with the original signal to obtain a direct current signal whose sign depends upon the sign of the original signal variations and applying said direct current to obtain an indication of its sign.

Arrangements for carrying out this method comprise means for obtainin from the signals of rectangular waveform a comparison signal of constant sign irrespective of the sign of the signal variations and arrangements for combining the comparison signal of constant sign with the original signal in a differential rectifier device, and a direct current indicating instrument sensitive to direction of current flow therethrough to which the differential output is applied.

The invention will be better understood from the following description taken in conjunction with the accompanying drawing in which Figures 1 and 3 show diagrammatically two embodiments of the invention, and

Figure 2 shows several curves referred to in the description.

. 2 The descriptionwill be given of the invention as applied to a radio'blind approach system.

Referring to Figure 1, the arrangements according to the invention comprises a rectifier bridge RB comprising four rectifiers r! 74 connected as shown, the output circuit of the amplifier valve V being in series with a resistance R2 and the indicating instrument A across one diagonal of the bridge. Th primary winding" L of a transformer T is connected acrossthe other diagonal of the bridge. The secondary winding of the transformer T is centre-tapped at P so that two equal'but oppositely phased alternating voltages appear across the two halves of the secondary Winding relative to the point P. These two portions of the secondary winding are connected in push-pullfashicn to a difierential rectifier system comprising rectifiers WI and W2. The parallel feed for the 'rectifiers WI and W2 is provided by the alternating voltage across the resistance R2 and A in series.

Referring now to Figure 2 of the drawing, curve a and curve I) show graphically dot and dash signals respectively, which would be obtained, one on each side of the approach path after demodulation from the radio carrier-of the received signals. Curves c and d show the results after the direct current component has been removed from thesignals represented by a and b respectively. This direct current component is removed by means of the resistance RI and capacity Cl which form a filter which passes all frequencies at and above the dot and dash frequency. The curves 0 and d therefore represent the unrectified current flowing through the resistance R2 and instrument A and the rectifier bridge in series with the latter. It will be observed that the two signals represented by c and d of Figure 2 are similar but Whose variations are of opposite signs. Curve c Figure 2 represents the voltage produced by the full-wave rectifier bridge across its load, winding L of transformer T, and is independent of whether dots or dashes are being received, that is, it is the same for both currents represented by c and d but the corresponding voltages across R2 will be as shown in c or 41 according as dots or dashes prevail in the received signal. Curve I Figure 2 represents the rectified wave in one half of the secondary winding of the transformer T since the transformer acts to pass substantially all frequencies but zero frequency, i. e. direct current. The wave in the other half of the secondary winding of the transformer T will be. curve 1 reversed about the zero line. I

The resistance 32 is preferably adjusted to such a value that the voltage across it has approximately the same amplitudes as that across each half of the transformer secondary winding. Thus one rectifier WI or W2 receives the difference between the voltage across R2 and that across one half of the secondarywinding of T this diilerence being substantially zero. The other of the two rectifiers WI or W2 receives the sum of the voltages across R2 and its associated half of the secondary Winding of T. Thus the indicating instrument receives a series of pulses,

representing the dot signals and will be in one direction or the other according to the sign of the Variations of signal 0 or d applied across R2. When the meter is suitably damped a steady deflection on one side or the other of zero is obtained according as dot signals or dash signals prevail thereby giving an indication as to which side of the approach path the receiver is located. When on the approach path a signal of constant amplitude or continuous dash is received and the rectifier system is not energised since the filter formed by RI' and CI does not pass any direct current, and no deflection of the indicating instrument can occur due to signal reception.

It will beobserved that the system according to the invention does not depend for its reliability upon an exact balance between rectifiers or other components.

Referring now to Figure 3, an alternative embodiment of the invention is shown. The transformer of the arrangement shown in Figure l is replaced by a resistance-condenser network. The bridge rectifier Works into the load resistance R3 and the direct current is removed by the two equal condensers C4 and C5 in conjunction with the equal resistances R4 and R5 respectively.

These elements provide the push-pullinput to the differential rectifiers WI and W2 and the operation is similar to that described in connection with Figure 1, a repetition of it being considered not necessary in connection with Figure 3.

Whilst two embodiments of the invention have been disclosed by way of example, other embodiments which fall within the scope of the invention as defined in the appended claims will occur to those skilled in the art.

What is claimed is:

1. In an arrangement for distinguishing visually between two signals of the same rectangular current wave form but whose variations are of opposite signs which comprises means for developing from either signal a comparison signal current of constant sign, means for combining this comparison signal current with th original signal current to obtain a direct current signal whose sign depends upon the sign of the original signal variations, and means for deriving from this direct current signal an indication of its sign.

2. In an arrangement for distinguishing visually between two signals of the same rectangular waveform but whose variations are of opposite sign, means for obtaining from the signals of rectangular wave form a comparison signal of constant sign irrespective of the sign of the signal variations and arrangements for combining the comparison signal of constant sign with the original signal comprising a differential rectifier device, and a direct current indicating instrument 4 approach path is defined by the intersection of two overlapping lobes complementary signals being transmitted by the two radiations comprising means for obtaining from the received sign depending upon the sign of the original signal variations, and a direct current indicating instrument sensitive to the sign of the current flowing therethrough to which the said direct current is applied.

4. Arrangements as claimed in claim 2 wherein said means for obtaining a comparison signal comprises a rectifier bridge across one diagonal of which the said signals of rectangular waveform are applied and a load impedance across the other diagonal of the bridge from the terminals of which the comparison signals of constant sign are obtained.

5. In an arrangement for distinguishing visually between two signals of the same rectangular wave form but whose variations are of opposite sign, means for obtaining from the signals a comparison signal of contant sign irrespective of the sign of the signal variations, said means for obtaining a comparison signal comprising a rectifier bridge across one diagonal of which said signals of rectangular wave form are applied and a load impedance across the other diagonal of the bridge from the terminal of which the comparison signal is obtained; means 'for combining the comparison signal of constant sign with the original signals comprising a differential rectifier device, said rectifier device having a resistance and a direct current indicating instrument sen sitive to direction of current flow therethrough included in series with the rectifier bridge and in parallel with the arms of the difierential rectifier.

6. Arrangements as claimed in claim 5 wherein said load impedance comprises a transformer, the secondary Winding of which is centre-tapped and the two halves of said secondary winding feed respective arms of the differential rectifier. I

7. In an arrangement for distinguishing visually between two signals of the same rectangular wave form but whose variations are of opposite sign, means for obtaining from the signals a comparison signal of constant sign irrespective of the sign of the signal variations, said means forobtaining a comparison signal comprising a rectifier bridge across one diagonal of which said signals of rectangular wave form are applied and a load impedance across the other diagonal of the bridge from the terminals of which the comparison signal is obtained, said load impedance comprising a resistance, means for combining the comparison signal of constant sign with the original signals comprising a differential rectifier device, means for applying the voltage changes at the ends of said resistance to the respective arms of the differential rectifier device, said rectifier device having a resistance and a direct current indicating instrument sensitive to direction of current flow therethrough included in series with the rectifier bridge and in parallel with the arms of, the differential rectifier device.

8. Arrangements as claimed in claim 7, wherein said last mentioned means comprises series connected capacity and resistance combinations in shunt across the respective ends of said load resistance and a reference point.

JEFFREY DENNIS WESTON. 

